Conventional methods for forming a conductive circuit include mainly an etching method, a screen printing method, a vacuum deposition method, a spin coating method and the like. As a substrate, various materials such as glass, epoxy, polyimide and silicon wafer are used depending on the use thereof.
The etching method is to cover a substrate with a conductive film and further form a conductive circuit by using a photo engraving method for leaving only the required conduit circuit of the conductive film by removing the other unnecessary portion of the conductive film than the circuit portion thereof by dissolving it in an etching solution.
The screen printing method is to form a conductive circuit by printing a pattern on the substrate with a conductive metal paste.
The vacuum deposition method is to form a conductive circuit on a wanted position of the substrate by mounting a mask on the substrate and vacuum-depositing a conductive metal thereon.
Currently, the above mentioned methods are widely used, but they are facing such problems as a complicated process, a relatively long process time, a loss of raw materials, a low yield rate, a high cost and an environmental problem. Particularly, they may cause a serious environmental problem in the course of disposing an etching solution, a washing solution, a photosensitive film and the like. A recent tendency is to exclude substances causing environmental contamination from raw materials used in the plating process for the reason of an environmental problem resulting from them. Therefore, the fact is that it is very difficult to control the required properties.
Further, electronic apparatus become more and more compact and multi-functional. Electronic components such as a resistor and a condenser mounted on the substrate are also manufactured in a very small size so that they are used as an element of a multi-layer substrate. Accordingly, a wiring circuit of the printing circuit substrate is also required to be highly minute and precise.
Recently, as the above mentioned problems have been raised and a conductive circuit pattern of a printing circuit substrate as required by the industry has got to be extremely various and complicated, many studies on development of a method for forming a circuit pattern by using an inkjet printer are being conducted.
The method for forming a conductive circuit by using an inkjet printer is to form a conductive circuit by discharging a conductive ink onto the substrate from the head of the inkjet printer according to a pattern designed by a computer system.
The method for forming a conductive circuit by using an inkjet printer can solve all the said problems and further has an advantage that a circuit can be easily designed thanks to rapid advancement of computer systems and peripheral devices thereof. In addition, it enables various types of products required by end-users to be manufactured in a small quantity and supplied to end-users at a low cost so that it can provide a method for lowering prices of expensive electronic products.
Despite the said advantages, a method for forming a conductive circuit by using an inkjet has not been widely used yet. The reason for this situation resides in the conductive ink. The conductive ink should meet largely two requirements. Firstly, it should have an electrical property, that is to say, the required conductibility, and its adhesive property to the substrate should be good. Secondly, it should satisfy properties of the ink enabling an inkjet printing work to be performed, e.g., viscosity, surface tension, stability and the like.
In order to meet the said requirements, various methods have been being studied.
U.S. Pat. No. 5,114,744 discloses a method for forming a conductive circuit. The ink composition used by this method includes stickers, additives and one or more solvents. The ink is transferred into the selected pattern by a continuous type of inkjet system. Subsequently, the metal powder gets to adhere to the substrate by stickers according to the pattern and any excessive metal powder is removed. And, a circuit pattern is formed by heating the substrate, pattern and metal powder at a temperature enough to dissolve the metal powder. The ink provided by this invention is not a conductive ink, but an adhesive ink. This invention is of a method for making a pattern of the adhesive ink on the substrate, jetting the metal powder onto it, removing the metal powder distributed outside the pattern, and melting the metal powder on the pattern by heating it. This invention is not suitable for the inkjet system because the viscosity of stickers of the adhesive ink enabling the metal powder to adhere to the pattern is high. Moreover, it has a disadvantage that because it should be treated at a high temperature enabling the metal powder to be molten, a substrate material should be also selected from materials stable at a high temperature.
More importance has been recently being attached to the usability of a universal substrate material in selection of a substrate material. For example, this is because in the case that a circuit board for an automation device, a camcorder and the like has to be movable with it being bent and in the case that the circuit board has to be bent when a component is inserted into it and a component is configured on it, a flexible PCB should be used as a substrate that can meet the requirements for such cases, that is to say, a suitable circuit substrate should be selected depending upon the used apparatus.
Such substrate materials include typically a polymer film material such as polyimide or polyester.
However, in the case of such polymer film, the process temperature range is limited because decomposition of polymer and deviation of a dimension may occur when it is baked at high temperature.
Also, Japanese Laid-open Patent Publication No. (Hei) 10-183207 describes an ink for inkjet printing in the form of an independent dispersed liquid of metal ultrafine particles which enables an electrode of a flat panel display (FPO) to be formed.
This patent publication introduced a technology for capturing silver fine powder into the solution by using the gas atmosphere phase evaporation method in order to manufacture fine powder, then obtaining a dispersed liquid of metal fine particles, then manufacturing an ink which can be jetted, and thereby forming a conductive circuit. However, this method requires extra devices for electronic beam plasma or laser induced heat so that it takes a high cost. Moreover, in the case of using such metal fine particles, it is very difficult to disperse them independently in the solvent so that it is difficult to maintain a high concentration of metal colloid phase. As a result, stability including preservative quality is not sufficient. Also, since a production amount of metal ultrafine particles is small, it is not easy to produce them on a large scale so that its use cannot avoid being limited to some special uses.
In addition, studies on manufacture of a metal paste and a conductive ink for forming a conductive circuit by using organic metal compounds have been recently being actively conducted.
For example, a method for forming a conductive circuit by using an organic silver solution and an inkjet printer was introduced in the reference literature titled Liquid Ink Jet Printing with MOD Inks for Hybrid Microcircuits (Teng, K. F., and Vest, R. W., IEEE Transactions on Components, Hybrids and Manufacturing Technology, 12(4), 545-549, 1987). However, it is pointed out that the solid matter thereof is small so that the coated film is very thin and the required conductibility fails to be obtained.
Korean Laid-open Patent Publication No. 2000-75549 discloses a method for forming a conductive film and a conductive circuit by baking a mixture of metal powder with an organic metal compound at a low temperature (300˜350° C.) by using a screen printing method. However, in the case that the conductive ink is manufactured by mixing such organic metal compound with the metal powder, a process that the metal powder should be dispersed is included, and it has a disadvantage that the viscosity should be lowered to enable the ink to be jetted.
This disadvantage is pointed out in the reference literature titled “Dispersion and Stability of Silver Ink” (B. Y. Tat. M. J. Edirisinghe, 2002), which mentions a conductive ink as manufactured by using three kinds of dispersing agents to disperse it stably.
In addition, there are a considerable number of related literatures, but all of them say that the ink should be manufacture through a dispersing process. For manufacturing an ink containing metal fine powder, it is a key point to make a stabilized colloid. However, it is very difficult to obtain a stabilized colloid, and in order to obtain it, a special dispersing process, that is to say, a milling process is required. Through this dispersing process, it is very difficult to obtain a constant result every time the work is conducted. If the work for maintaining an average particle size below a given level and adjusting all physical properties correctly meeting is not performed properly, particles with a great average particle diameter clogs a nozzle of the inkjet head to cause defective printing so that it gets to be impossible to form a conductive circuit continuously.